Field of the Invention
The present invention relates to a method for producing a multi-layer polyethylene resin foam sheet.
Description of Related Art
A polyethylene resin foam sheet has been utilized in a wide variety of fields. For example, a polyethylene resin foam sheet is now suitably used in various applications, such as shock absorbing materials (e.g. packaging materials and packing materials) and partitions, because of its flexibility and cushioning property. However, a polyethylene resin foam sheet, when composed only of a polyethylene resin foam layer, fails to show environmental stress cracking resistance (hereinafter referred to as ESCR). Namely, when the sheet is exposed simultaneously both to a liquid containing a surfactant or the like chemical and to external stresses, environmental stress cracking or rupture of the sheet is apt to be caused. Thus, there has been a strong demand for improving ESCR of polyethylene resin foam sheets.
To cope with this demand, there is proposed a laminated foam sheet in which an oriented polypropylene film (hereinafter referred to “OPP film”) is laminated on a surface of a polyethylene resin foam layer. Since the OPP film has high ESCR, especially high resistance to surfactants, the laminated foam sheet has excellent ESCR and yet shows excellent flexibility and shock absorbing property inherent to its polyethylene resin foam layer. Moreover, owing to the OPP film, the laminated foam sheet excels in aesthetics in terms of surface smoothness and gloss and in stiffness. With a view toward further improving ESCR and aesthetics of the laminated foam sheet, a proposal is made to laminate OPP films on both sides of a polyethylene resin foam layer.
As a method for preparing an OPP film-bearing laminated foam sheet, there is proposed a method (first method) in which an OPP film is first applied with a coating of an adhesive, the resulting OPP film being subsequently laminated on one or both sides of a polyethylene resin foam layer. The first method is excellent in productivity since both sides of the polyethylene resin foam layer may be simultaneously laminated with the OPP films. However, the first method in which an adhesive must be used has a problem because, when the laminated foam sheet is immersed in a liquid containing a surfactant or the like chemical, the adhesive gradually dissolves in the liquid so that inter-layer peeling is liable to occur between the OPP films and the polyethylene resin foam layer.
As a method for preparing a laminated foam sheet, a method (second method) is also known which includes kneading polypropylene (PP) and polyethylene (PE) using an extruder to obtain a molten resin composition, and extruding the molten resin composition between a polyethylene resin foam layer and an OPP film to bond the OPP film and the polyethylene resin foam layer to each other with the resin composition serving as an adhesive layer (see, for example, Japanese Kokai Publication JP-A-2007-284126).
When the second method is adopted for producing a laminated foam sheet in which an OPP film is laminated on each side of the polyethylene resin foam layer, the molten resin composition is extruded and applied onto one of the both sides of the polyethylene resin foam layer to form thereon the adhesive layer, with which the OPP film is brought into contact. It is inevitable to separately conduct the steps of laminating the OPP films on both sides of the polyethylene resin foam layer. Accordingly, the second method has a problem of poor production efficiency because an increased number of the production steps are required. Further, the laminated foam sheet is likely to warp when one side of the polyethylene resin foam layer is heated by application with the molten resin composition. Warp of the laminated foam sheets causes difficulty in handling and deterioration of sealing property (especially when the laminated foam sheets are used as a packing material) and appearance.